1. Field of the Invention
The present invention relates an image projection apparatus, a method for controlling the image projection apparatus, and a computer-readable storage medium.
2. Description of the Related Art
High-pressure mercury-vapor lamps are widely used as light sources for projectors. The high-pressure mercury-vapor lamp causes vaporized mercury (light-emitting material) sealed in an arc tube to emit light by utilizing an arc discharge between a pair of electrodes arranged in the arc tube. When the projector is powered off, the vaporized mercury returns to its liquid state. However, liquid mercury can stick to the pair of electrodes in this process, resulting in undesirable short circuit between the electrodes. This phenomenon, referred to as “mercury bridging”, can be a cause of failure of the projector to light up.
Mercury bridging occurs due to the following two properties: 1) liquefied mercury is likely to stick to a low-temperature portion; and 2) because the electrode unit is more prone to temperature change than the inner bulb unit (arc tube), the temperature of the electrode unit is likely to be lower than the temperature of the inner bulb unit after the projector is powered off. In particular, there is correlation between the temperature of the electrodes and lamp power (electric power supplied to the electrodes). Accordingly, when the projector is powered off in a state where the lamp power is low, difference in temperature between the electrodes and the inner bulb unit widens, and probability of occurrence of mercury bridging increases by leaps. There are conventionally-known techniques that lessen the probability of occurrence of mercury bridging by narrowing the temperature difference between the electrodes and the inner bulb unit by applying after-cooling. The after-cooling is, more specifically, continuing cooling the lamp over a predetermined period of time even after the projector is powered off. For example, Japanese Patent No. 4070420 discloses a technique for preventing mercury bridging at power-off of a projector by powering off the projector after maintaining a lamp-power-low state for a predetermined period of time.
However, a cooling fan keeps rotating in the projector that needs the after-cooling even after a user that has completed a projector job powers off the projector to put it away. Therefore, the projector is disadvantageous in the following ways: the user cannot put away the projector immediately; noise of the fan interferes with conversation; and the user can mistake the after-cooling as a failure and make a customer complaint. Although the technique disclosed in Japanese Patent No. 4070420 does not require after-cooling, this technique maintains the lamp power in the low state until the temperature of the inner bulb unit drops to the boiling point of mercury or lower. Accordingly, this technique is disadvantageous in that the projector is not brought to a complete stop immediately after the projector is powered off, and, furthermore, the projector cannot be put away immediately because it takes long time for the temperature of the inner bulb unit to drop to the boiling point of mercury or lower. Thus, the conventional techniques have sacrificed user's convenience.
Therefore, there is a need to provide an image projection apparatus capable of improving user's convenience, and a method for controlling the image projection apparatus.